A point-to-point Optical Wireless Link (OWLink) using collimated beams of light to wirelessly transfer high speed data (=>100 Mbps) is currently being developed by applicant. As depicted in FIG. 1, a uni-directional OWLink 10 consists of one unit 13 including a transmitter with DSP-based control electronics 40 that transfers information through an optical beam 16 to a second unit 15 including a receiver. In a bi-direction communication system, as shown in FIG. 2, a bidirectional OWLink unit 17, called a port in this application, contains both a transmitter 13 and a receiver 15. The OWLink port 17 also contains a data interface 19 that is adapted to be connected to a remote data source and sink of data that is transmitted across the optical wireless link. The data interface 19 can be a serial interface such as Ethernet, USB, RS232, telephone modem, or a parallel interface such as a computer bus (PCI, PCMCIA, Cardbus). The source and sink of data could be a computer, local area network (LAN), personal digital assistant, LAN switch, or any device that can generate or receive data.
The transmitter 13 includes a light source such as a laser diode 11 that emits a collimated or anisotropic light beam 12, a data interface input 21 connected to electronics for modulating the light with data, a 2-axis analog micromirror 14 for selectively directing the reflected light beam 16, and a photodetector 18 for detecting when light is returned to the transmitter during the initial alignment. The micromirror 14 can selectively point the collimated light source 12 in any direction within its field of view (FOV), such as within 5 degrees with respect to a nominal direction.
The receiver 17 consists of a data photodiode 20 with optics 22 for collecting the impinging light, four (4) “positioning” photodiodes 24 surrounding the collection optics 22, and a retro-reflective (corner cube) element 26. Retro-reflective elements alone are well known in the art—examples include street sign material and bicycle reflectors. The data photodiode 20 is connected to a data interface output 23. The “positioning” photodiodes 24, each being equal distance from the center of the collection optics 22, provide information on how well an incoming beam 16 is aligned to the collection optics 22 by comparison of the intensity of incoming light on each photodiode 24. The use of the collection optics 22 increases the area of light that can be directed to hit the data photodiode 20 while restricting the FOV of received light. Ideally, the FOV of the transmitter 13 and receiver 15 are the same. The retro-reflective element 26 has the property of reflecting incoming light back upon itself, thereby returning part 28 of the incoming light signal not incident to the optics 22 in the direction of its transmitter. The teachings of pending commonly assigned patent application Ser. No. 60/234,081, entitled, “Optical Wireless Network with Direct Optical Beam Pointing” discussing retroreflectors for acquisition of a point-to-point optical wireless link is incorporated herein by reference.
The transmitter 13 and receiver 15 for a port 7 do not necessarily need to be physically next to each other. However, they share a data interface 19, such as the same Ethernet cable, RS232 cable, or phone line.
The method described above for establishing a single point-to-point optical wireless connection incurs difficulties when multiple receivers 15 lie within a transmitter's FOV, since the retro-reflector 26 at each receiver will return light the same transmitter. This would be the case for a hub consisting of a collection of multiple ports in close proximity. In addition, it is important that the process of establishing new links interferes as little as possible with already established links. Moreover, in the process of establishing a new link between a hub and a remote port, the hub must be able to direct the remote port to connect to one of its unused ports with an appropriate FOV.